For a proper diagnosis, more information is needed: Boot an emergency Linux disc (like the installer in its "try before installing" mode) and run the Boot Info Script. This will produce a file called
RESULTS.txt with lots of output. Post it to a pastebin site and post the URL to your document here.
Without a proper diagnosis, there are two shot-in-the-dark approaches that might get you up and running again:
- Download the USB flash drive or CD-R version of my rEFInd boot manager and prepare a medium from the image. Disable Secure Boot (if it's active) and boot the medium. It should give you options to boot both Windows and Ubuntu. If both options work, boot to Ubuntu and install the Debian-package version of rEFInd. It should then take over your boot manager duties.
- Boot to Linux (using either an emergency disk in EFI mode or rEFInd to boot the Linux on your hard disk) and run the Boot Repair tool. This program will re-install GRUB on your hard disk, which should begin working again.
Note that the Boot Repair approach involves more of a leap of faith than the rEFInd approach. With rEFInd, the USB drive or CD-R boot serves as a proof of concept; if you can boot from the USB drive, you can be certain that rEFInd will work if the install to the hard disk is successful. With Boot Repair, though, there's no way to test its installation before it's done. In your case, it is at least unlikely to do any further damage.
If both methods fail, post the URL that Boot Repair produces. This is essentially the same output that Boot Info Script produces.